The invention relates to a CMOS current controlled ring oscillator (VCO) and particularly to a method and system for obtaining a well-controlled output range over varying process.
The VCO is an oscillator whose frequency is proportional to an externally applied voltage. VCO's are commonly used in phase-locked loop (PLL) designs. A block diagram of a basic PLL system is shown in FIG. 1. The elements of the system are a phase detector (12), a loop filter (14), and a VCO (16) interconnected to form a feedback system. The phase detector compares the phase of the input signal Vs(t) with the output frequency Vo(t) of the VCO and generates an error voltage Vd(t) corresponding to the difference. The error voltage signal Vd(t) is then filtered by the loop filter and applied to the control terminal of the VCO in the form of an error voltage Ve(t) to control its frequency of oscillation.
A problem that has plagued VCO designs in the past has been that of maintaining a constant VCO Ve(t) error voltage characteristic over the wide process variations that are inherent in the CMOS devices (i.e. changes in the manufacturing process, such as changes in doping levels and lithography variations). Process variations can cause a huge shift in the gain of a delay element that forms the ring oscillator. This in turn changes the gain constant, tuning slope, and the frequency (FIG. 2). Good PLL designs require a knowledge and control of several loop parameters--one of which is the VCO gain. Thus, the VCO circuit needs to be de-sensitized to the process variations so that the PLL dynamics are not disturbed.
A patent assigned to National Semiconductor Corporation and issued to Rasmussen, U.S. Pat. No. 5,061,907 discloses a VCO that includes a multistage ring oscillator, a voltage-to-current converter, process compensation circuitry, and a trip-point compensation circuit. This VCO achieves immunity from process variations by the use of the process compensation circuitry and the trip-point compensation circuitry. The process compensation circuitry in Rasmussen responds to the error voltage input signal to provide a current dump output signal that is dependent on transistor strength, i.e., dependent on process variation. The trip-point compensation circuit in Rasmussen generates a net current created by subtracting the process compensation current from the current generated by the voltage-to-current converter, and then steers a variable percentage of that net current to the ring oscillator in accordance with the strength of the N- and P-channel transistors in the device.
The output currents from both the voltage-to-current converter and the process compensation circuitry in Rasmussen are dependent on the error voltage input signal and the relative strengths of the N and P channel devices. However, under certain conditions where the N and P channel devices are particularly strong and the error voltage is at a particularly high level, an inappropriate current dump output signal can exist. This condition exists when the process compensation current (current dump) becomes too high and thereby too much current is starved from the ring oscillator via the trip point compensation circuitry. The present invention avoids this inappropriate current dump output signal condition by never allowing the ring oscillator current level to become too low.
Another drawback to Rasmussen is that the process compensation current (current dump) will vary with VCC supply voltage. Thus, changes in VCC supply voltage may cause the current dump to become too low or too high. The present invention avoids this dependency on the VCC supply voltage by using only the error voltage and process strength to establish the proper current levels.
A patent assigned to National Semiconductor Corporation and issued to Jelinek et al., U.S. Pat. No. 5,331,295 discloses a VCO that includes a multistage ring oscillator that includes a plurality of series-connected current-starved inverter stages. The VCO utilizes a first current source to provide a substantially constant current independent of process variations and a second current source to provide a variable current that varies in response to process variations. An attenuator, responsive to the VCO's input voltage signal (typically from a phase-locked loop filter) provides a control current signal to the ring oscillator. The attenuator receives a supply current created by subtracting the second current from the first current, and utilizes a differentiation subcircuit that attenuates the supply current in response to changes in an input voltage to produce a current control signal that sets the current level in the ring oscillator's cells. The frequency of oscillation of the ring oscillator is determined by the control current signal.
A drawback to Jelinek is the requirement of the first current source to provide an accurate current that is independent of process variations. This first current source requires the use of complex circuitry and accurate resistor elements which increases the size and power consumption of the overall circuit. The present invention avoids the use of such a large and complicated current source.